Counterfeit Bill

Riddle:

A man buys a rope from a woman for $3.00 and hands the woman a $10 bill. The woman goes into the grocery store next door to get change. She returns and gives the man $7.00. After the man leaves, the clerk from the store comes and says, "Hey, that was a counterfeit bill you gave me." The woman gives the clerk a good bill. How much has the woman lost?